On the harmonic superspace geometry of $(4,4)$ supersymmetric sigma models with torsion
Starting from the dual action of $(4,4)$ $2D$ twisted multiplets in the harmonic superspace with two independent sets of $SU(2)$ harmonic variables, we present its generalization which hopefully provides an off-shell description of general $(4,4)$ supersymmetric sigma models with torsion. Like the action of the torsionless $(4,4)$ hyper-Kähler sigma models in the standard harmonic superspace, it is characterized by a number of superfield potentials. They depend on $n$ copies of a triple of analytic harmonic $(4,4)$ superfields. As distinct from the hyper-Kähler case, the potentials prove to be severely constrained by the self-consistency condition which stems from the commutativity of the left and right harmonic derivatives. We show that for $n=1$ these constraints reduce the general action to that of $(4,4)$ twisted multiplet, while for $n\geq 2$ there exists a wide class of new actions which cannot be written only via twisted multiplets. Their most striking feature is the nonabelian and in general nonlinear gauge invariance which substitutes the abelian gauge symmetry of the dual action of twisted multiplets and ensures the correct number of physical degrees of freedom. We show, on a simple example, that these actions describe sigma models with non-commuting left and right complex structures on the bosonic target.